Project Summary/Abstract. Adolescence is a unique developmental period that is accompanied by numerous cognitive, social, and emotional changes. Therefore, it is critical for youth to alter their thoughts, behaviors, and emotions flexibly. Although most studies have focused on cognitive flexibility, flexibility is also needed within emotional contexts. In particular, valence flexibility (i.e., switching between negative and positive emotions) may play a large role in socio-emotional development. Individuals must learn to switch between negative and positive emotions when perceiving, evaluating, and experiencing emotion. Difficulties engaging valence flexibility may manifest in symptoms such as persistent worry or prolonged sadness. The research objective of this functional magnetic resonance imaging (fMRI) study is to characterize normative developmental changes in behavior and neural circuitry engaged during valence flexibility across multiple levels of emotion processing (i.e., perception, appraisal, and evocation). The central hypothesis is that valence flexibility will increase with age and the developmental trajectories of valence flexibility will vary depending on the level of emotional processing. Moreover, developmental changes in valence flexibility will be observed in the interactions within and between brain networks engaged in task-switching (i.e., fronto-striatal), emotional processing (e.g., cortico- amygdala), and other large-scale brain networks (i.e., default mode network, DMN, salience network, SN). This R21 project is based on published work demonstrating developmental, neural, and individual differences in during cognitive flexibility, emotional processing, and affective flexibility (i.e., when switching between non- emotional and emotional content). Aim 1: To characterize the typical development of valence flexibility. Aim 2: To compare the developmental differences in valence flexibility across levels of emotional processing. Exploratory Aim 3: To delineate the relationship between measures of valence flexibility and socio-emotional processing, concurrently and one year later. Inside the MRI scanner, youth from late childhood to emerging adulthood (i.e., 9-20 years old) will complete three emotional paradigms involving selecting facial expressions (e.g., happy, sad/angry), appraising evaluative statements about themselves and someone else, and viewing evocative pictures. Valence flexibility will be measured when switching between negative and positive emotions relative to repeating emotions of the same valence. Reaction time cost and changes in neural circuitry will be measured. This neuroimaging study is innovative because it examines the development of flexibility within the emotional domain, provides an integrative perspective across different levels of emotion processing, and uses a unique approach to examine how neural networks interact when switching between negative and positive emotions. Identifying the normative developmental course of valence flexibility will provide key insights to future understanding of how perturbations affect well-being and may lead to internalizing disorders (e.g., anxiety and depression), which often emerge during the adolescent period.